The present invention hereby incorporates by reference, application Ser. No. 10/425,565, entitled “Clay Nanocomposites Prepared by In-situ Polymerization”, filed on Apr. 29, 2002.
Foamed polymers are found in applications ranging from packaging, insulation, cushions, adsorbents, to scaffolds for tissue engineering. The basic principle of foaming is to mix a blowing agent (typically a gas) into a polymer melt and induce a thermodynamic instability through a temperature or pressure change to nucleate bubbles of the blowing agent.
In this invention, supercritical CO2 (the critical temperature Tc: 31° C. and the critical pressure Pc: 73.8 bar or 1074 psi), a potential replacement of the traditional foaming agents (hydrocarbon or chlorofluorocarbon), was applied, The liquid-like solubility and gas-like diffusivity make it possible to dissolve sufficient CO2 in a polymer quickly. CO2 is low-cost, non-flammable, chemically benign, and environmentally friendly.
Recently, microcellular foams, characterized by cell sizes smaller than 10 μm and cell density larger than 109 cells/cm3, have drawn a great deal of attention and interest. It has been shown that by keeping the cell (or bubble) size uniformly less than 10 microns in diameter, one can greatly reduce material usage without compromising mechanical properties because the bubbles are smaller than the preexisting flaws in a polymer matrix.
The field of polymer/clay nanocomposites has grown rapidly in the past decade. In this work, nano-sized particles, nanoclays, are applied to modify the cellular foams in both batch and continuous extrusion foaming process. The results show that with the addition of a very small amount of nanoclay into the polymer matrix, the nanocomposites exhibit substantial increase in many physical properties, including mechanical strength (tensile modulus and strength, flexural modulus and strength), thermal stability, flame retardance, and barrier resistance. Smectite clays, such as montmorillonite (MMT), are of particular interest because they have a high aspect ratio (lateral dimension ˜200–500 nm, thickness <1 nm) and a high surface area. However, clay is hydrophilic in nature and incompatible with most polymers. To increase the compatibility and miscibility of clay in polymer, the clay surface is modified by an organic surfactant, typically ammonium cations with long alkyl chains.
Two idealized polymer/clay structures are possible: intercalated and exfoliated. Exfoliation involves extensive polymer penetration to disrupt the clay crystallite (tactoids), and the individual nanometer-thick silicate platelets are dispersed in the polymer matrix. If there is only limited polymer chain insertion in the interlayer region, and the interlayer spacing only expands to a certain extent without losing layer registry, then an intercalated nanocomposites is then formed.
Polymer foam is another area subject to intensive research. It is widely used for insulation, packaging, and structural applications, to name a few. Microcellular foam, which is characterized by cell size in the range of 0.1˜10 μm, cell density in the range of 109 to 1015 cells/cc, provides improved mechanical properties as well as increased thermal stability and lower thermal conductivity.
Cell nucleation and growth are two important factors controlling cell morphology. Particles can serve as a nucleation agent to improve heterogeneous nucleation. Some inorganic nucleation agents, such as talc, silicon oxide, kaoline, etc., are widely used. A fine dispersion of these nucleation agents can promote formation of nucleation center for the gaseous phase. Although a detailed explanation of the heterogeneous nucleation mechanism is still not available, the size, shape, and distribution, and surface treatment of particles have great influences on the nucleation efficiency. In this work, we developed a new polymer nanocomposite foam preparation technology to create polymer foams with controlled cell structure. In addition, clay may further improve the foam properties, e.g., mechanical and barrier properties, as well as fire resistance.